The atmospheric carbon dioxide (CO2) measurements made at Mauna Loa Observatory, Hawaii since
1958 provide strong evidence for human alteration of the environment (Fig. 17). The data through 1973 are
from Keeling et al. (1982), while data since 1973 are from the National Oceanic and Atmospheric
Administration (NOAA) program (Thoning et al. 1989).

Mauna Loa Observatory is located at an elevation of 3350 m on the flank of Mauna Loa volcano and is
an ideal site for carbon dioxide measurements. There is no nearby vegetation, and the prevailing nighttime
downslope winds give a representative sampling of mid-tropospheric air from the central North Pacific Ocean. Thus,
this record is taken as a reliable index of long-term carbon dioxide growth.

The average CO2 concentration increase at Mauna Loa during the 1980s and 1990s has been about
1.41.5 ppm yr-1, but with significant year-to-year variability in the growth rate (Fig. 17). These fluctuations in
growth rate appear to be strongly influenced by the state of the Southern Oscillation, with increases in the growth
rate evident during cold episodes (e.g., 1988/89, 1995/96) and decreases evident during warm episodes (e.g.,
1982/83, 1991/92). The very strong 1997/98 El Niño appears to have slowed the rate of increase by the middle
of 1997.